Such a system is known from U.S. Pat. No. 4,490,833. The system is used for the examination of welds during construction of a storage tank. These tanks are also referred to as vertical storage tanks as the design basically consists of a vertically positioned cylinder, although the diameter can be up to 100 meters or even larger.
The vertical part of the tank (the shell or wall) is constructed of multiple metal plates. Typically the size of these plates is 10 meters in horizontal direction and 3 meters in vertical direction, but other sizes can be used as well. The thickness of the used plates depends on the design of the tank (diameter, height, plate material etc.) and purpose (pressure, substance to be stored, temperature etc.). Typically the thickness of the vertical plates varies from relatively thick at the bottom to thinner at the top. The lower vertical plates are thicker, for example 25 to 30 mm thickness, to withstand the weight of the plates above and the fluid pressure of the medium stored in the tank. The upper plates can be thinner, for example 10 mm or thinner, because less weight is on top, a lower fluid pressure exists at higher elevation and to limit the amount of required material. Alternatively, all plates of the tank wall could have the same thickness, depending on the design and circumstances.
Depending on the substance to be stored inside the tank the material of the tank plates can be standard low alloy carbon steel (e.g. for crude oil or oil products), or an alloyed steel that is suitable for the stored product or circumstances (e.g. 9% Ni steel for storage of LNG, Liquefied Natural Gas, at −162° C.). Some storage tanks are made of non ferritic material like aluminium.
Typically all welds (horizontal and vertical) between the plates of the tank shell (wall) must be examined to ensure the integrity of the welds.
Some welds are more difficult to access or to examine, for example the weld connecting the tank wall and the (horizontal) bottom plates of the tank floor. These welds and the welds between the plates of the tank bottom plates are not considered here. The welds to be inspected are the welds between the metal plates.
A disadvantage of the known system is that films are used allowing only one exposure a time. This is slow and requires chemical processing of the film.
Digital radiography is also known as such and covers a variety of technologies, comparable to the medical sector, like:                Image plates that have a layer sensitive to X-rays (like a phosphor plate) that temporary stores a latent image. The latent image can be read by a dedicated scanner device and then the image is stored on a computer;        Flat panel, utilising a material that converts the X-rays into digital signals (directly or indirectly), for example an amorphous silicon panel, connected to a computer on which the digital signals are stored as image.        
The above technologies are only suitable for static exposure, meaning that both the X-ray source and the detector (film, image plate, or flat panel) have to be stationary relative to each other and to the object during the exposure of the detector to radiation. The maximum size of a film, image plate or flat panel is typically about 30 to 40 centimeters which determines the maximum weld length that can be examined in one exposure. To ensure that the entire length of the weld is examined it is required that consecutive exposures overlap, for example 5 centimeters, so the effective exposed length is always shorter than the size of the film or detector. As a result much time is involved in handling and positioning of the equipment to a next stationary position for making a new exposure. As explained for films and image plates also additional processing (development, readout) is required.
Conventional radiography with films uses static exposures and requires operator handling on both sides of the tank wall (for the X-ray source and for the films), for exposure of each individual film.
Systems for digital radiography mainly use static exposures, for example on flat panel detectors. Therefore operator handling would be required on both sides of the tank wall, similar to conventional radiography.
For radiographic weld examination strict requirements apply to the image quality of the resulting image to ensure proper detection and evaluation of possible welding imperfections. These requirements are available in (inter)national codes and standards and are, for example, resolution and contrast. To show that the system meets the requirements it is mandatory that image quality indicators are attached to the weld and are visible on the resulting image, according to the codes and standards. For digital systems additional requirements can apply. Not all available digital systems are able to fulfil the requirements for weld examination of storage tanks.
Due to radiation safety regulations no other personnel is allowed during radiographic examination, a so called exclusion zone must be established. Depending on the situation the exclusion zone can extend over part or even the complete tank. Obviously, this limits the construction progress (welding etc.). Typically the welding and construction activities are performed during the day shift while the radiographic examination is performed during the night shift. It could be beneficial to have multiple construction/welding shifts working consecutively around the clock (24 hours per day) but this interferes with the radiographic examination due to radiation safety. In many construction projects the examination of welds is on the critical path of the construction process, so the progress of the whole project depends directly on the progress of the examinations.
Some welding imperfections can be related to the welding process, welding materials and settings of the welding system. For example the speed at which the welder, or the welding machine, proceeds during welding can influence the weld quality. It is important that information about the weld quality is provided to the welder as soon as possible, to enable the welder to adjust any parameter of the welding process if required. Such feedback on weld quality can consist of, for example, the presence of small imperfections which are not of direct relevance for the quality of that weld but still indicate a sub-optimal setting of the welding parameters. If the welding parameters are not adjusted then possibly in the next welds larger imperfections could occur which are not allowed and the weld has to be repaired, possibly over larger distances. Such repairs require additional work (removing part of the weld, re-welding and re-examination) which could ultimately interfere with the scheduled construction progress. So, if the feedback on weld quality is provided earlier to the welder it may be possible to adjust the welding parameters timely and prevent the occurrence of larger, not allowable imperfections. The early feedback to the welder is only possible if the progress of the examination of the welds can keep up with the welding progress and the examination can be performed as close as possible to the welding.